Pva lubricant for polyvinyl chloride

ABSTRACT

The heat seal characteristics of extruded oriented polyvinyl chloride film are improved by the addition of low molecular weight polyvinyl acetate homopolymers to a conventional extrusion mix and thereafter carrying out extrusion operations.

United States Patent 1 Matthews [451 Sept. 18, 1973 PVA LUBRICANT FORPOLYVINYL CHLORIDE [75] Inventor: Stephen B. Matthews, Waynesboro,

[73] Assignee: Reynolds Metals Company,

Richmond, Va.

[22] Filed: Apr. 12, 1972 [2]] Appl. No.: 243,474

[56] 5 References Cited UNITED STATES PATENTS 2,682,484 6/1954 Thomas260/899 X 9/1945 Britton et al 260/899 X l2/l966 Weisman 260/328 PrimaryExaminer-John C; Bleutge Assistant Examiner-C. J. Seccuro Attorney-JohnF. C. Glenn et al.

57 ABSTRACT The heat seal characteristics of extruded oriented polyvinylchloride film are improved by the addition of low molecular weightpolyvinyl acetate homopolymers to a conventional extrusion mix andthereafter carrying out extrusion operations.

8 Claims, N0 Drawings DESCRIPTION OF THE PRIOR ART Polyvinyl chloridepolymers are extremely well known in the art and, in fact, have foundgreat acceptance for a wide variety of uses including the manufacture ofpolyvinyl chloride films. An extremely important method for preparationof polyvinyl chloride films which is employed commercially todayinvolves socalled extrusion processes. In processes of this type,polyvinyl chloride together with conventional additives is forcedthrough a die or other restrictive orifice to yield films having desiredcharacteristics. it is also well known in the art that in the extrusionof polyvinyl chloride films, it is necessary to employ lubricants toreduce the friction l heat generation in the prefused and moltencompound, as well as to reduce melt viscosity and melt temperature andto enhance the smooth melt flow and prevent sticking and burning on theinternal parts of the extruder. There are many lubricants known in theart which admirably accomplish all of the above obj ectives. However,aparticular problem arises when attempting to manufacture polyvinylchloride films which are clear or transparent in that although thelubricant is desirable during the extrusion phase of the operation, ingeneral, it is undesirable in the finished product.

The reason why lubricants can generally be stated to e undes able in. te in he film s due t e t that in order for lubricants to functioneffectively, they must be completely or partially incompatible with thepolymer, i.e., the particular lubricant must be able to migrate throughthe polymer in order to provide the lubricity which is necessary in theextruding operation. However, as has heretofore been stated, althoughthis property is deemed to be essential during the extrusion operation,nevertheless, certain problems do arise once the final. film has beenproduced. It has been discovered that conventional lubricants tendtomigrate to the surface of the finished film and their presence canadversely affect several desirable film properties, particularly inclear polyvinyl chloride films. One of the more critical propertieswhich is adversely affected by the heretofore employed lubricantsistheheat seal range which is defined as the temperature span over which twolayers of the film will fuse without melting. Conventional lubricantsmigrate to the film surface forming a heat barrier to the heat seal,thereby severely affecting the heat seal range.

Although the aforementioned migration of lubricants to the surface ofthe film does occur in all types of extruded polyvinyl chloride films,its adverse effects do,

not become highlighteduntil the film is oriented by conventionaltechniques such as stretchingln a nonoriented film, the sites, formigration are less and the subsequent effects do not appreciably narrowthe heat seal range. However, there are many applications for polyvinylchloride film which require that it be oriented in order'to impartdesirable characteristics to the film, particularly when it is desiredtoproduce heat shrinkable film. to be employed in conventional packagingoperations. For reasons which are not completelyundetstood, when apolyvinylchloride film has been oriented by stretching, the problemofmigration of thelubricant tof the surface of, the film becomes muchmore severe and eirperie ce has, shown that it has an adverse effect onheat, seat characteristics of the desired material. In fact, ith-asbeenshown that the heat, seal range of an 2 oriented polyvinyl chloride filmwhich initially is between 30-40 F may be reduced to a range of l0 F ina period of time of two months or less.

DESCRIPTION OF THE INVENTION It has now been discovered that clearpolyvinyl chloride films can be produced by conventional extrusiontechniques which will retain their initial heat seal characteristicsafter they have been oriented either uniaxially or biaxially by theinclusion in a conventional polyvinyl chloride extrusion formulation ofa lubricant consisting essentially of a low molecular weight homopolymerof polyvinyl acetate. It has been found that the homopolymers ofpolyvinyl acetate provide the lubrication which is necessary during theextrusion operation without seriously affecting the characteristics ofthe finished polyvinyl chloride film. The reason why polyvinyl acetatehomopolymers function in a manner to produce the outstanding results isnot completely understood, but the simple fact remains that orientedextruded polyvinyl chloride films containing polyvinyl acetate as alubricant do not have their heat seal characteristics adverselyaffected, and, in fact, in some cases it has been observed that the heatseal strength is actually improved over comparable films produced in thesame manner but without the use of the novel lubricant of thisinvention. r

This invention is conveniently practiced by adding small amounts ofpolyvinyl acetate to a conventional polyvinyl chloride extrusionformulation, said formulation containing polyvinyl chloride plus theconventional additives well known in the art. The amount of lowmolecular weight polyvinyl acetate which is incorporated -into thepolyvinyl chloride is governed 'by many factors which will behereinafterset forth. As has heretofore been stated, the problem in the art has notbeen to provide a lubricant to avoid the problems during the extrusionoperation, but rather, to provide a lubricant which would not createproblems in the finished film. In this regard, it should be immediatelyapparent that in the manufacture of clear polyvinyl chlo' ride films, itis necessary that the various additives conventionally used includinglubricant do not impart a discoloration or a haziness to the finalproduct. For this reason it has been found that the amount of polyvinylacetate homopolymer which should be employed should range from 0.5 toabout 2'weight percent based on the polyvinyl chloride resin. It shouldbe immediately apparent that higher concentrations of polyvinyl acetatecan be used if one considers merely the lubrieating qualities ofpolyvinyl acetate. Thus, the higher the concentration of polyvinylacetate, the greater will be the amount of lubrication which occursduring the extrusion operation.'However, increasing amounts of polyvinylacetate will affect the quality of the final film and tend-to make ithazy which, in turn, limits its applicability in those packagingoperations where a clear film is desired in order that the object beingpackaged will be visible without any adverse aesthetic effects. For thisreason, it has been found that a maximum of 2 weight percent of thepolyvinyl acetate should be used in order to avoid problems with regardto theclarity of the film. On the other hand, if less than about 0.5weight percent of polyvinyl acetate is employed, sufficient lubricationwill not be present during the extrusionv operation.

The polyvinyl acetate homopolymers which are employed as lubricants forthe production of polyvinyl chloride films in accordance with theteachings of this invention are well known in the art and arecommercially available by a number of manufacturers. In general,polyvinyl acetate homopolymers can be produced so that the averagemolecular weight thereof ranges anywhere from about 4,300 to 100,000 andhigher. Because of the difficulty of accurately measuring the molecularweight in the lower end of the molecular weight range, the art hasrefrained from ascribing numerical values of average molecular weight tothese polymers. Instead the polyvinyl acetate art has developed suchthat molecular weights of the varying homopolymers are described byreference to the softening point of the particular homopolymer. Ingeneral, the higher the softening point, the higher the molecular weightof the polyvinyl acetate homopolymer. In accordance with the acceptedmethod of description in the art, the homopolymers with which thisinvention is concerned are those which have a softening point (asdetermined by the modified ball and ring method) ranging from about 44 Cto about 86.5 C and preferably from 60 to 70 C. A particularly preferredlow molecular weight vinyl acetate homopolymer is one which has asoftening point of 66 C. A material of this type is commerciallyavailable under the trade name AYAA (Union Carbide Corporation).

In a preferred embodiment of this invention, it is desired to use lowmolecular weight polyvinyl acetate homopolymers which have been mixedwith approximately 5% of fine particle size polyvinyl chloride in orderto minimize blocking. As is well known in the art, low molecular weightpolyvinyl acetate homopolymers which are supplied in the form of pelletshave a tendency to stick together due to blocking and sintering duringstorage, especially at temperatures above 70 F. Such phenomenon rendersprocessing with this resin more difficult. In order to avoid theaforementioned problems of blocking and sticking, polyvinyl acetatehomopolymer powders are admixed with about 5% by weight of a fineparticle size polyvinyl chloride. A material of this type is also knownin the art and is commercially available under the trade name AYJV(Union Carbide Corporation).

As has heretofore been stated, this invention is conveniently practicedby adding a low molecular weight polyvinyl acetate homopolymer to aconventional polyvinyl chloride extrusion mix, and then carrying outconventional extruding operations and conventional stretching ororientation operations. As is well known in the art, polyvinyl chlorideextrusion mixes can contain a wide variety of additives to impartspecific properties or characteristics to the finished polyvinylchloride film. These additives include, for example, the use ofplasticizers in order to decrease the inherent rigidity of the polyvinylchloride. Typical plasticizers which are employed include tricresylphosphate, dioctyl phthalate, dibutoxyethyl phthalate, dioctyl adipate,epoxidized soy bean oil, etc. Other additives conventionally employedinclude heat stabilizers, the most common ones being alkaline earthcarbonates, hydroxides, oxides, and soaps. In addition, the extrusionformulation can contain dyes, pigments, diluent resins, fillers, etc.

To the above-described conventional polyvinyl chloride extrusion mix isadded the low molecular weight polyvinyl acetate homopolymers previouslydescribed in amounts ranging from about 0.5 to about 2 weight percentand the extrusion formulation is usually mixed in order to assureuniform blending of all the additives.

The extrusion melt is thereafter forced through conventional extrudingapparatus to produce the desired film and the film is thereafterstretched either uniaxially or biaxially in accordance with techniqueswell known in the art.

The following examples will illustrate the best mode contemplated forcarrying out the invention.

EXAMPLE 1 A polyvinyl chloride extrusion melt was prepared by blendingthe following:

Polyvinyl chloridel00 Dioctyl Adipate-l5 Epoxidized Soy Bean Oil-5 ButylTin Mercaptide l .5

*A polyvinyl acetate homopolymer having a softening point of 66C admixedwith about 5 weight percent powdered polyvinyl chloride, marketed byUnion Carbide Corporation.

The above was tumbled in a blender to about 250 F and thereafter tumbledin a cooling blender until the temperature dropped to about F. It wasthen passed through a conventional blown film extruder to form polyvinylchloride film. The extrusion opration was carried out smoothly with nosticking of the melt on the internal portions of the extruder therebyindicating the excellent lubricating properties of the polyvinylacetate. The finished film is then stretched in both directions toapproximately two and one-half times the original dimensions. Thefinished film is observed to have excellent heat seal characteristicswhich are not adversely affected upon storage.

EXAMPLE 2 The process of Example 1 is repeated with the exception thatthe polyvinyl acetate homopolymer employed is one which has a softeningpoint of 86.5 C. A product of this type is marketed by Union CarbideCorporation under the designation AYAT. The results of this exampleindicate that excellent lubricating qualities are obtained during theextrusion operation and the oriented film has excellent heat sealcharacteristics.

EXAMPLE 3 I The procedure of Example 1 is repeated with the exceptionthat the polyvinyl chloride extrusion melt has the followingcomposition:

Polyvinyl Chloride- I00 Epoxidized Soy Bean Oil-21 Ethylene BisOleamide0.23 AYJV*0;75

Liquid Barium Cadmium Phosphite Heat Stabilize- *A polyvinyl acetatehomopolymer having a softening point of 66 C admixed with about 5 weightpercent of polyvinyl chloride powder marketed by Union CarbideCorporation. The results of this example show that there is no problemwith sticking of the melt on the internal parts of the extruder therebyindicating good lubrication qualities. The finished film afterstretching in both directions shows excellent heat seal characteristicswhich do not substantially diminish over a period of time.

EXAMPLE 4 The procedure of Example 3 is repeated with the exception thatAYAF is employed. This material is a polyvinyl acetate homopolymerhaving a softening point of 770 C and marketed by the Union CarbideCorporation.

The results of this example indicate that this particular polyvinylacetate homopolymer works equally as well.

EXAMPLE 5 The procedure of Example 4 is repeated with the exception thatAYAA is employed. This material is a polyvinyl acetate homopolymerhaving a softening point of 770 C and marketed by the Union CarbideCorporation.

The results of this example indicate that this particular polyvinylacetate homopolymer works equally as passed through an extruder, theimprovement which comprises adding to said melt a low molecular weightpolyvinyl acetate homopolymer in amounts ranging from 0.5 to about 2.0weight percent based on the polyvinyl chloride.

2. The process of claim 1 wherein the low molecular weight polyvinylacetate homopolymer has a softening point of from about 44 to about 86.5C.

3. The process of claim 2 wherein the low molecular weight polyvinylacetate homopolymer has a softening point of from about 60-70 C.

4. The process of claim 2 wherein the polyvinyl chloride film issubjected to stretching operations.

5. The process of claim 3 wherein the low molecular weight polyvinylacetate homopolymer contains about 5 weight percent of finely dividedpolyvinyl chloride.

6. An extruded biaxially or uniaxially oriented polyvinyl chloride filmcontaining from about 0.5 to about 2.0 weight percent of a low molecularweight polyvinyl acetate homopolymer.

7. The film of claim 6 wherein the low molecular weight polyvinylacetate homopolymer has a softening point of from about 44 to about 865C.

8. The film of claim 6 wherein the low molecular weight polyvinylacetate homopolymer has a softening point of from about 60-70C.

2. The process of claim 1 wherein the low molecular weight polyvinylacetate homopolymer has a softening point of from about 44* to about86.5* C.
 3. The process of claIm 2 wherein the low molecular weightpolyvinyl acetate homopolymer has a softening point of from about60*-70* C.
 4. The process of claim 2 wherein the polyvinyl chloride filmis subjected to stretching operations.
 5. The process of claim 3 whereinthe low molecular weight polyvinyl acetate homopolymer contains about 5weight percent of finely divided polyvinyl chloride.
 6. An extrudedbiaxially or uniaxially oriented polyvinyl chloride film containing fromabout 0.5 to about 2.0 weight percent of a low molecular weightpolyvinyl acetate homopolymer.
 7. The film of claim 6 wherein the lowmolecular weight polyvinyl acetate homopolymer has a softening point offrom about 44* to about 86.5* C.
 8. The film of claim 6 wherein the lowmolecular weight polyvinyl acetate homopolymer has a softening point offrom about 60-70*C.